Development of a compound semiconductor device, in particular, a nitride semiconductor device, has been actively carried out to realize a semiconductor device having a high withstand voltage and a high output using features, such as a high saturated electron speed and a wide bandgap. As for the nitride semiconductor device, many reports on field-effect transistors, in particular, on a high electron mobility transistor (HEMT), have been disclosed. Specifically, as a GaN semiconductor device containing GaN, an AlGaN/GaN HEMT in which GaN is used as an active layer (electron transit layer) and AlGaN is used as an electron supply layer has attracted attention. In the AlGaN/GaN HEMT, strain is generated in AlGaN by the difference in lattice constant between GaN and AlGaN. By piezoelectric polarization generated by this strain and spontaneous polarization of AlGaN, a high concentration two-dimensional electron gas (2DEG) may be obtained. Hence, a semiconductor device having a high withstand voltage and a high output may be realized.
In the GaN semiconductor device, since an inexpensive and a large-diameter Si substrate may be used as a substrate for crystal growth, a significant reduction in manufacturing cost may be advantageously expected.
In the GaN semiconductor device, it has been confirmed that for example, when GaN is formed as a nitride layer functioning as an active layer, as the thickness of this GaN is increased, the number of defects is decreased, and as a result, the quality is improved. As a particular example, the results obtained through investigation on GaN layers having thicknesses of 200 nm and 600 nm by an x-ray rocking curve method (XRC method) are depicted in FIGS. 1A and 1B.
However, although the manufacturing cost may be reduced, when a thick GaN active layer is formed on a Si substrate so as to obtain an active layer having a small number of defects and a high quality, the following problems may arise.
For example, as depicted in a part (a) of FIG. 2, a thick GaN layer 103 is formed on a Si substrate 101 with an AlN buffer layer 102 interposed therebetween. The lattice constant of Si is larger than that of GaN, and the coefficient of thermal expansion of GaN is larger than that of Si. Hence, when the temperature is decreased after the active layer 103 is formed, a downward convex warp is generated by thermal contraction as depicted in a part (b) of FIG. 2. This warp is increased as the thickness of the GaN layer 103 is increased, and as a result, a crack is liable to occur. This phenomenon indicates that improvement in dielectric breakdown withstand voltage of the device expected by an increase in thickness of a nitride layer originally having a wide bandgap and high insulating properties and improvement in quality caused by a decrease in dislocation density are to be restricted.
As a method to overcome the above problem, that is, as a method for increasing the thickness of a nitride layer while the generation of warp and crack is suppressed, for example, there have been known a stepwise AlGaN buffer in which several AlGaN layers having different Al composition ratios are laminated to each other and a strained layer superlattice (SLS) buffer in which the structure formed by alternately laminating a GaN thin film and an AlN thin film many cycles is inserted under a GaN electron transit layer. In both of the above structures, since a large internal compressive strain is generated in the GaN electron transit layer, an intense tensile strain of the entire nitride layers generated in a temperature decrease step performed after the film formation is cancelled out, so that the generation of warp and crack is suppressed. However, since the buffer structures as described above each inevitably become complicated, and the total film-forming time is increased, this increase in time may be one of causes to inhibit improvement in throughput. In addition, the consumption amounts of raw materials, such as expensive organic metal materials, are also increased, and hence, this increase becomes a bottleneck of a mass production process.
The followings are reference documents:    [Document 1] Japanese Laid-open Patent Publication No. 2012-23314,    [Document 2] Japanese Laid-open Patent Publication No. 2007-67077,    [Document 3] Japanese Laid-open Patent Publication No. 2005-317909, and    [Document 4] A. Y. Polyakov, et al. “Electrical and optical properties of Fe-doped semi-insulating GaN templates”, Applied Physics Letters, Vol. 83, Number 16 (2003).